Thin-film el device, and its fabrication process

Abstract

The invention aims to provide, without incurring any cost increase, a thin-film EL device comprising a multilayer dielectric layer formed of a lead-based dielectric material by a solution coating-and-firing process, which has solved problems including light emission luminance drops, luminance variations and changes of light emission luminance with time, thereby achieving high display quality, and a process for the fabrication of the same. The object is accomplished by forming a patterned electrode layer on an electrically insulating substrate and constructing thereon a dielectric layer having a multilayer structure wherein lead-based dielectric layers formed by repeating the solution coating-and-firing process plural times and a non-lead-based, high-permittivity dielectric layer are stacked, the uppermost surface layer of the dielectric layer having a multilayer structure being the non-lead-based, high-permittivity dielectric layer.

Description

[0001] 1. Technical Field[0002] This invention relates to a thin-film EL device having at least a structure comprising an electrically insulating substrate, a patterned electrode layer on the substrate, and a dielectric layer, a light-emitting layer and a transparent electrode layer stacked on the electrode layer.[0003] 2. Background Art[0004] EL devices are now practically used in the form of backlights for liquid crystal displays (LCDs) and watches. The EL devices work on a phenomenon in which a substance emits light at an applied electric field, viz., an electro-luminescence (EL) phenomenon.[0005] The EL devices are divided into two types: dispersion type EL devices having a structure wherein electrode layers are provided on the upper and lower sides of a dispersion of light-emitting powder in an organic material or porcelain enamel, and thin-film EL devices having a thin-film light-emitting substance sandwiched between two electrode layers and two thin-film insulators on an elec...

AI Technical Summary

Benefits of technology

[0082] By use of the non-lead-based, high-permittivity dielectric layer, it is thus possible to easily achieve the effect of the invention on prevention of the diffusion of the lead component into the light-emitting layer while the effective relative permittivity decrease is minimized.

[0083] In this connection, the inventor's studies have revealed that when such a non-lead-based dielectric layer, especially a perovskite structure material, is used, it is of importance that its composition is such that the ratio of A site atoms to B site atoms in the perovskite structure is at least 1.

[0084] To be more specific, all perovskite structure non-lead-based dielectric materials as mentioned above may crystallographically contain lead ions at the A site. Taking a BaTiO.sub.3 composition as an example, consider the case where the starting composition for the formation of a BaTiO.sub.3 layer is such that Ba that is the A site atom is deficient with respect to Ti that is the B site atom, as expressed by Ba.sub.1-xTiO.sub.3-x. Since an excessive lead component exists in the lead-based dielectric layer forming the BaTiO.sub.3 layer, the Ba deficient site in the BaTiO.sub.3 is easily replaced by the excessive lead component, yielding a (Ba.sub.1-xPb.sub.x)TiO.sub.3 layer. When a light-emitting layer is formed on the BaTiO.sub.3 layer in such a state, no sufficient effect on prevention of the diffusion of lead is obtained because the light-emitting layer comes in direct contact with the lead component.

[0085] It is thus preferred that the composition of the perovskite structure non-lead-based dielectric layer should at least be shifted to an A site excess side from the stoichiometric composition. As can be inferred from this explanation, even when the composition of the perovskite structure non-lead-based dielectric material is shifted to an A site excess side from the stoichiometric composition, there is a significant if remote possibility that the portion of the non-lead-based dielectric layer in the vicinity of the interface with respect to the lead-based dielectric layer may react with a part of the lead component, because the perovskite structure non-lead-based dielectric material may crystallographically be substituted by the lead component. For this reason, the non-lead-based dielectric layer should preferably have a certain or greater thickness. According to the inventor's experimental studies, this thickness should be 0.1 .mu.m or greater, and preferably greater than 0.2 .mu.m.

[0086] Like the perovskite dielectric materials, in the case of tungsten bronze type dielectric materials as typified by SBN: (Sr.sub.1-xBa.sub.x)Nb.sub.2O.sub.6, whose composition is represented by the chemical formula: A.sub.xB.sub.xO.sub.15, wherein the A ion can be replaced by Pb, it is desired that the cation at the A site be present in an amount of equal to or more than the stoichiometry.

[0087] For the formation of the non-lead-based dielectric layer while its composition is under full control, it is preferable to make use of a sputtering process or the solution coating-and-firing process because the composition can be well controlled.

Problems solved by technology

Known for long, the dispersion type EL device has the advantage of ease of fabrication; however, it has only limited use on account of low luminance and short service life.

However, for these thin-film EL devices, a structural problem remains unsolved.

The problem is that since the insulator layers are each formed of a thin film, it is difficult to reduce to nil steps at the edges of the pattern of the transparent electrode, which occur when a large area display is fabricated, and defects in the thin-film insulators, which are caused by dust, etc. occurring in the production process, resulting in a destruction of the light-emitting layer due to a local dielectric strength drop.

Such defects offer a fatal problem to display devices, and become a bottleneck in the wide practical use of thin-film EL devices in a large-area display system, in contrast to liquid crystal displays or plasma displays.

Although the use of this thick- film dielectric layer leads to a problem that the effective voltage applied to the light-emitting layer drops, this problem can be solved or eliminated by using a high permittivity material for the dielectric layer.

However, it is still difficult to sufficiently smooth down the surface of a dielectric layer fabricated by an ordinary thick-film process.

For this reason, the sintering of the thick-film dielectric layer does not proceed to a sufficient extent, yielding an essentially porous layer.

Since the consolidation process proceeds through a solid phase reaction of ceramic powder having a certain particle size distribution, abnormally sintered sites such as abnormal crystal grain growth and macropores are likely to occur.

This results in problems such as a decrease in effective light-emitting area because an electric field cannot be effectively applied to the portions of the light-emitting layer formed on non-flat portions of the substrate, and a decrease in light emission luminance because local non-uniformity of thickness causes a local dielectric breakdown of the light-emitting layer.

Furthermore, locally large thickness fluctuations cause the strength of an electric field applied to the light-emitting layer to locally vary too largely to obtain any definite light emission voltage threshold.

However, the polishing of a large-area substrate for display or other purposes is technically difficult to achieve, and is a factor for cost increases.

The addition of the sol-gel step is another factor for cost increases.

When a thick-film dielectric layer has abnormally sintered sites which may give rise to asperities too large for removal by polishing, they cannot be removed even by the addition of the sol-gel step, which causes a drop of manufacturing yield.

It is thus very difficult to use a thick-film dielectric material to form a light emission defect-free dielectric layer at low cost.

In consideration of heat resistance and a reactivity problem with respect to the dielectric layer, the substrate used for the formation of such a thick-film dielectric layer is limited to alumina or zirconia ceramic substrate; it is difficult to rely on inexpensive glass substrates.

The substrate meeting such conditions is obtained only with much technical difficulty, and is yet another factor for cost increases.

This, too, is a factor for cost increases.

However, when the multilayer dielectric layer is formed by a solution coating-and-firing process, using a lead-based dielectric material as the dielectric layer material, a light-emitting layer to be formed on the dielectric layer can react with the lead component of the dielectric layer, giving rise to some practically unfavorable problems such as initial light emission luminance drops, luminance variations, and changes of light emission luminance with time

Such an excessive lead component precipitates easily from within the dielectric layer under thermal loads after the formation of the dielectric layer, especially thermal loads in a reducing atmosphere.

If a light-emitting layer as mentioned later is formed directly on this dielectric layer, there would then be light emission luminance drops and considerable adverse influences on long-term reliability through the reaction of the light-emitting layer with the lead component and contamination of the light-emitting layer with movable metal lead ions.

Even when lead oxide is not reduced to metal lead by the reducing atmosphere in particular, the incorporation of the lead oxide component in the light-emitting layer causes lead oxide to be reduced by electron bombardments due to high electric fields within the light-emitting layer with the result that the released metal ions have an adverse influence on reliability.

However, any satisfactory effect on prevention of the diffusion of lead is hardly obtained because of minute surface defects in the lead-based dielectric layer or the surface roughness thereof, or the local surface roughness of the non-lead-based dielectric layer due to the deposition of dust or the like ascribable to fabrication steps.

This may otherwise result in a local decrease or deterioration in the luminance of the light-emitting layer due to the local diffusion of the lead component.

For this reason, the use of such a non-lead-based dielectric layer causes EL device drive voltage to become too high to obtain practical utility.

When a light-emitting layer is formed on the BaTiO.sub.3 layer in such a state, no sufficient effect on prevention of the diffusion of lead is obtained because the light-emitting layer comes in direct contact with the lead component.

A thickness exceeding 16 .mu.m results in cost increases because the number of repetition of the solution coating-and-firing process becomes too large.

However, much technical difficulty is generally encountered in forming a dielectric layer having a relative permittivity of 1,500 or greater, using the solution coating-and-firing process.

A functional thin film like an EL light-emitting layer cannot possibly be formed and used on such a dielectric layer.

No particular limitation is imposed on the thickness of the light-emitting layer; however, too large a thickness leads to a driving voltage rise whereas too small a thickness causes a light emission luminance drop.

With the prior art, problems such as the reaction of the lead component in the dielectric layer with the light-emitting layer and the diffusion of lead are unavoidable.

Images